Claude Marceau

Advances in intense femtosecond laser filamentation in air

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.

Simultaneous detection and identification of multigas pollutants using filament-induced nonlinear spectroscopy

The authors report on an approach for simultaneous monitoring of multigas pollutants based on fluorescence emission of trace gases, induced by the filamentation of intense femtosecond laser pulses in air. The high intensity inside a filament can dissociate the gas molecules into small fragments which emit characteristic fluorescence. This method is illustrated for simultaneously sensing atmospheric trace gases, methane and acetylene. The spectra of an “unknown” mixture were analyzed by using a genetic algorithm, showing good concentration agreement with the experimental results within an error of 25%.

Communication: Two stages of ultrafast hydrogen migration in methanol driven by intense laser fields

Hydrogen migration in methanol induced by an intense laser field (0.2 PW/cm(2)) is investigated in real time by a pump-probe coincidence momentum imaging method. The observed temporal evolution of the kinetic energy spectra reveals that there are two distinctively different stages in the hydrogen migration processes in the singly charged methanol: ultrafast hydrogen migration occurring within the intense laser field ( approximately 38 fs) and slower postlaser pulse hydrogen migration ( approximately 150 fs).

Toward remote high energy terahertz generation

Remote terahertz (THz) generation from a two-color femtosecond laser-induced filament in air was experimentally demonstrated. A record of remote THz emission at 16 m was achieved. THz pulse energy more than 250 nJ in the frequency range below 5.5 THz was recorded; this is two orders of magnitude stronger than that from single-color excitation. Back-scattered nitrogen (N2) fluorescence signal remotely measured with a lidar is linearly proportional to the THz emission, which would provide a more practical method to characterize the THz pulses.

Characterization of terahertz emission from a dc-biased filament in air

We demonstrate that the terahertz emission from a dc-biased filament can be regarded as a sum of an elliptically polarized terahertz source (generated by a filament without external electric field) and a linearly polarized terahertz source induced by the external electric field applied to the filament. The peak frequency and linewidth of the linearly polarized terahertz source are related to the average plasma density of the filament.

High energy terahertz emission from two-color laser-induced filamentation in air with pump pulse duration control

Two-color laser-induced femtosecond filamentation was employed to generate high energy terahertz emission in air with high energy pump. By controlling the pump pulse duration, more than four times enhancement in terahertz pulse energy has been obtained when compared with a Fourier transform-limited pump. Multiple filaments’ dynamics might be responsible for the terahertz enhancement. Superbroadband terahertz pulse with energy up to 2 μJ was generated using loose focusing condition, while the maximum terahertz pulse energy in the frequency range below 5.5 THz was around 60 nJ.

Elliptically polarized terahertz emission in the forward direction of a femtosecond laser filament in air

Elliptically polarized terahertz emission from a femtosecond laser filament in air in the forward direction was discovered by using a wire grid polarizer and electro-optic sampling technique. The generation mechanism could be through four-wave optical rectification or second-order optical rectification inside the filament zone where the inversion symmetry of air is broken.

Ultrafast birefringence induced by a femtosecond laser filament in gases.

We demonstrate that a femtosecond-laser filament in both molecular and atomic gases is birefringent for a copropagating probe pulse. Any input-probe polarization is decomposed into two orthogonal components, the optical axis being in the pump polarization direction. In molecular gases, the birefringence is mainly due to the delayed rotational molecular-wave packet; the probe pulse thus experiences several revivals in time. The two probe components end up spatially separated in the far field. In atomic gases such as argon, the effect is weaker and is attributed to the instantaneous electronic cross-phase modulation.

Polarization rotation due to femtosecond filamentation in an atomic gas

The linear-to-elliptical transformation of a 400 nm femtosecond-probe pulse in the birefringent filament in argon of an 800 nm linearly polarized femtosecond-pump pulse is studied numerically and experimentally. The rotation of the probe elliptical polarization is the largest in the high-intensity filament core. With propagation, the rotated radiation diffracts outward by the pump-produced plasma. The transmission of the analyzer crossing the probes polarization is maximum at the pump-probe angle of 45 degrees and gives equal values for each pair of angles symmetrically situated at both sides of the maximum.

Terahertz emission from a dc-biased two-color femtosecond laser-induced filament in air

The generation of terahertz (THz) emission from a dc-biased two-color femtosecond laser-induced filament in air was systematically investigated. A polarization analysis demonstrated that the THz emission could be the sum of two components: one generated by two-color laser-induced filamentation and the second induced by the external dc electric field. The first component is mostly from four-wave mixing process and a transient transverse electric current under the action of the external dc field could be responsible for the second THz emission.